awards_and_contracts.md: update purchase orders example

[odscjen]

closes #1694

I've retained using csv table examples rather than json as I think it helps to see the values in a table format. The new example isn't a very good one for double counting risk as the initial contract has a maximumValue rather than a fixed value so as well as a table showing the initial contract and one showing the purchase order transactions I've also included a third example which shows how putting it all in Contract.value could lead to double counting.